Mobile terminals such as mobile phones, Personal Digital Assistants (PDAs) and the like are powered by rechargeable batteries. The battery of the mobile terminal is charged with electrical energy by a separate charging device. Typically, a separate contact terminal is formed on the outer side of each of the charging device and the battery, and the charging device and the battery are electrically connected to each other through contact between the two separate contact terminals.
However, in such a contact-type charging scheme, the contact terminals protrude outwardly, and thus are easily contaminated by foreign substances. As a result, battery charging is not correctly performed. Further, battery charging may not be correctly performed when the contact terminals are exposed to moisture.
In order to solve the above-mentioned problems, a wireless charging technology or a non-contact charging technology has recently been developed and used in many electronic devices.
The wireless charging technology uses wireless power transmission and reception, and corresponds to, for example, a system in which a battery is automatically charged if the battery is just put on a charging pad without connecting the mobile phone to a separate charging connector. Typically, the wireless charging technology is generally known to be used in a wireless electric toothbrush or a wireless electric shaver. The wireless charging technology can improve a waterproof function because it can be used to wirelessly charge the electronic devices. The wireless charging technology can improve the portability of the electronic devices because it does not require a wired charger. Therefore, it is expected that technologies related to the wireless charging technology will be significantly developed in the coming age of electric cars.
The wireless charging technology largely includes an electromagnetic induction scheme using a coil, a resonance scheme using resonance, and a Radio Frequency (RF)/microwave radiation scheme for converting electrical energy into a microwave and transmitting the microwave.
Until now, the wireless charging technology using the electromagnetic induction scheme has been the mainstream. However, recently, experiments in which power is wirelessly transmitted over a distance of several tens of meters by using microwaves have been successful at home and abroad. Accordingly, it is expected that an environment in which all electronic devices can be wirelessly charged anywhere and anytime will be realized in the near future.
A power transmission method using electromagnetic induction corresponds to a scheme for transmitting power between a primary coil and a secondary coil. When a magnet approaches the coil, an induced current is generated. A transmission side generates a magnetic field by using the induction current, and a reception side generates electrical energy by inducing a current according to a change in the magnetic field. This phenomenon is called a magnetic induction phenomenon, and the power transmission method using this phenomenon has excellent energy transmission efficiency.
With respect to the resonance scheme, in 2005, Professor Soljacic of the Massachusetts Institute of Technology (MIT) published a system in which electricity is wirelessly transferred from a charging device to a device to be charged by using a power transmission principle of the resonance scheme known as a coupled mode theory even when the device to be charged is several meters away from the charging device. A wireless charging system of the MIT research team employs the physics concept called “resonance” in which when a tuning fork oscillates at a particular frequency, a wine glass next to the tuning fork will oscillate at the same frequency. The MIT research team caused an electromagnetic wave containing electrical energy to resonate instead of causing sound to resonate. It is known that the resonant electrical energy does not affect surrounding machines and human bodies differently from other electromagnetic waves because the resonant electrical energy is directly transferred only to a device having a resonance frequency and its unused part is reabsorbed into an electromagnetic field instead of spreading into the air.
Meanwhile, a wireless power transmitter needs the development of a method for detecting a wireless power receiver. When the wireless power receiver is placed on the wireless power transmitter for the sake of wireless charging, the wireless power transmitter detects the placed wireless power receiver and can transmit charging power to the placed wireless power receiver. Specifically, when the wireless power receiver is not placed, the wireless power transmitter does not transmit charging power. Only when the wireless power receiver is placed, the wireless power transmitter can transmit charging power. Therefore, there is a need to develop a method in which a wireless power transmitter detects a wireless power receiver.